Britain and the US last night faced fresh allegations of abuses after a British terror suspect said an SAS soldier had interrogated him for three hours while an American colleague pointed a gun at him and threatened to shoot him.

The allegation is contained in a new dossier detailing repeated beatings and humiliation suffered by three Britons who were captured in Afghanistan, then held in Guantánamo Bay for two years, before being released in March without charge.

Rhuhel Ahmed, one of the "Tipton Three", claims in the 115-page dossier that shortly after his capture in November 2001 he was interviewed in Afghanistan by a British interrogator who said he was from the SAS. Mr Ahmed alleges he was taken by US guards to be interrogated by the British officer in a tent. "One of the US soldiers had a gun to his head and he was told if he moved they would shoot him," the report says. The SAS officer pressed him to admit he had gone to Afghanistan to fight a holy war. Last night the Ministry of Defence said it would investigate the allegation.

A spokesman said: "The British army follows the rules laid out in the Geneva convention and soldiers are told to follow that. It is not permissible to point guns at people's heads during interrogation. We would investigate if any allegation of that nature is made."

The dossier, based on two months of interviews by the men's lawyers, provides the first full account by the three Britons of their ordeal as terror suspects.

Details of the experiences of Shafiq Rasul, Asif Iqbal and Rhuhel Ahmed, all from the same small Midlands town, are revealed today by the Guardian, and will be formally released this afternoon in the US. The three Britons allege they were repeatedly beaten, shackled in painful positions during interrogations and subjected to sleep deprivation. On one occasion, Mr Iqbal recalled: "I was left in a room and strobe lighting was put on and very loud music. It was a dance version of Eminem played repeatedly."

Mr Rasul said he was asked: "If I wanted to get surface-to-air missiles from someone in Tipton, who would I go to?"

In an echo of the abuse scandal at Abu Ghraib jail in Baghdad which shamed Washington, the three Britons, held as illegal enemy combatants by the US, say they were photographed naked and subjected to anal searches unnecessarily, after being shackled for hours.

The three claim their interrogators, from a phalanx of US intelligence agencies including the CIA, accused them of being in a video shot in 2000 alongside Osama bin Laden, the al-Qaida leader, and Mohamed Atta, the leader of the September 11 attack. At the time one of the three was working in a Currys electrical store in the Midlands and two others were in trouble with the British police. Despite this, all three say the pain they were in and ill treatment led them confess to being in the video.

The dossier also alleges complicity by Britain in their treatment. The three challenge a claim by the Foreign Office junior minister, Chris Mullin, who in the Commons said no Briton had complained of their treatment in Guantánamo. Mr Iqbal says a British embassy official took down a two-page list of alleged abuses, while the two others say they made their complaints orally. Mr Rasul says he was interrogated by British personnel up to seven times, with MI5 officers questioning the Britons repeatedly.

On June 4 last year Tony Blair told the Commons: "Information is still coming from people detained there ... that information is important."

In the dossier the Britons say the level of mental illness among detainees is higher than admitted by the US. The Tipton Three say guards told them that a fellow British detainee, Moazzam Begg, still imprisoned in Guantánamo, had been kept in isolation and "was in a very bad way". They say that Jamil el-Banna, of London, was so traumatised that "mentally, basically, he's finished".

Mr Banna is a Jordanian citizen with refugee status in Britain, but the government refuses to represent him. It also refuses to represent another Londoner, Bisher al-Rawi, originally from Iraq, who lived in Kingston, south-west London.

Lawyer Gareth Peirce said the report showed Britain's complicity in the human rights abuses at Guantánamo: "The [British government] attitude displayed the hypocrisy of the public face in the UK saying we're doing all we can and the private face there in Guantánamo involved up to their elbows in the oppression."

Nine Britons were imprisoned in Guantánamo without charge or access to a lawyer. The Tipton Three were among five released in March, who were questioned on arrival in Britain before being released. Four Britons remain in Guantánamo, as well as four British residents.